384 
POPULAR SCIENCE REVIEW. 
The coating of the sac or cell-wall ( h ) consists of a very fine 
membrane of cellulose ; within which are a variety of substances 
known as cell-contents. Two of these are invariably present in 
all young growing cells — water, and a mucilaginous, more or 
less granular substance known as protoplasm (p ) ; situated 
within which, but varying in position, generally almost close to 
one side, is a nearly transparent body consisting of denser, 
almost solid protoplasm, the nucleus ( k ). The structure and 
composition of each of these parts must now be examined 
somewhat more in detail. 
The Cell-wall is always of uniform chemical composition, its 
constitution being C 6 H 10 O 5 , identical with starch and dextrine. 
Though perfectly continuous and destitute of any pores or 
orifices that can be detected even by the highest powers of 
the microscope, the cell-wall is, nevertheless, permeable to 
fluids, the cell-sap passing into the cell by the process of osmose ; 
that is, the passage through a permeable diaphragm of the less 
dense of two fluids of different specific gravity, which are 
separated by the diaphragm. The substance of the cell- wall 
is secreted from the protoplasm which it contains ; but, accord- 
ing to Nageli, Sachs, and other competent authorities, its 
growth in thickness does not depend, as was at one time sup- 
posed, on the formation of new consecutive layers, each within 
those already in existence, but on the “ intussusception ” or 
interposition of fresh particles or molecules of cellulose, which 
penetrate into every part of the cell-wall from the protoplasm. 
The cell-wall is perfectly colourless, and when thin transparent, 
so as to reveal the internal structure of the cell. 
The Protoplasm is the essential life-giving portion of the 
cell. It is not homogeneous in its consistency ; the layer nearest 
the cell-wall is somewhat denser, and forms a kind of skin or 
bladder enveloping the interior portion, and was termed by 
Mohl the “ Primordial Utricle ” (Primordial-schlauch), a term 
which is still generally applied to it. The Nucleus* already 
described is present in the early stage of the cells of all higher 
organisms without exception; but that it is not an essential 
ingredient is shown by its absence from Unicellular Algae and 
the cells of some other lowly organisms. Very commonly the 
nucleus contains other smaller bodies of a similar character 
within it, the Nucleoli. The protoplasm is often absent from one 
or more cavities in the interior of the cell, termed vacuoles, 
which are occupied by the cell-sap. The protoplasm may be 
contracted and separated from the cell-wall by the application 
of iodine and dilute sulphuric acid; a drop of iodized solution 
of chloride of zinc causes the cell-wall to assume a beautiful 
* Called by Schleiden and the older writers, the Cytoblast. 
